As shown in FIG. 1, an induction motor (I) is generally illustrated by a typical T type steady-state equivalent circuit, wherein: Rs represents a stator resistance, Rr represents a rotor resistance, Lσs represents a leakage inductance of the stator winding, Lσr represents a leakage inductance of the rotor winding, Lm represents an excitation inductance, s represents a slip, self-inductance of stator winding Ls=Lm+Lσs, self-inductance of rotor winding Lr=Lm+Lσr, Us represents a phase voltage, Is represents a phase current, and Im represents an excitation current.
In practice, an induction motor (as a controlled object) achieves a high performance vector control of the rotor flux linkage orientation by a power converter, and generally adopts a T-II type steady-state equivalent circuit, as shown in FIG. 2, wherein: Rs, Rr, s, Us and Is have the same meanings as described above, thus, no further details thereof will be given; Lσ represents a total leakage inductance; L′m represents an excitation inductance; I′m represents an excitation current.
While the T type and T-II type steady-state equivalent circuits are totally equivalent to each other, the T-II type steady-state equivalent circuit is constructed with a constant total rotor flux linkage. A relationship between the two types of steady-state equivalent circuits is:
            Maintaining      ⁢                          ⁢              R        s            ⁢                          ⁢      constant        ;                  L        σ            =                        (                      1            -                                          L                m                2                                                              L                  s                                ⁢                                  L                  r                                                              )                ·                  L          s                      ;                      L        m        ′            =                        L          m          2                          L          r                      ;                  R        r        ′            =                                    (                                          L                m                                            L                r                                      )                    2                ·                  R          r                    
The T-II type steady-state equivalent circuit will be expressly illustrated below. For simplicity, the stator resistance, the rotor resistance, the total leakage inductance, the excitation inductance, and the excitation current in induction motor parameters of the equivalent circuit are respectively represented by Rs, Rr, Lσ, Lm, Im, as shown in FIG. 3.
Among the aforesaid parameters, the stator resistance Rs, the total leakage inductance Lσ and the rotor resistance Rr can be measured when the induction motor is at standstill. The excitation parameters (i.e., the excitation current Im, the excitation inductance Lm) of the induction motor can be measured only when the induction motor is under no-load rotary test. The principle of the no-load test is as follows: when a shaft end of the induction motor is disconnected from any load, the induction motor rotates approximately at a synchronous speed, the slip s≈0, and an impedance of a rotor circuit is infinite such that the rotor circuit can be regarded as an open circuit. And then the induction motor shown in FIG. 3 may be converted into an equivalent circuit shown in FIG. 4. A three-phase symmetric alternating voltage Us with a specific frequency f is input into the equivalent circuit, wherein the three-phase alternating voltage Us corresponds to the phase current Is. At this time, the input phase current Is is totally used for excitation, that is, Is≈Im. A reactive power Q of the induction motor can be calculated by using a sample current Is and a voltage signal Us, and the inductance of the stator winding Ls of the induction motor can be calculated by an equation Ls=Q/(2πf·Im2), wherein the inductance of the stator winding Ls contains the excitation inductance Lm and the leakage inductance Lσ, and then the excitation inductance Lm can be calculated by subtracting the leakage inductance Lσ from the inductance of the stator winding Ls, thereby a corresponding air gap flux linkage can be calculated, i.e., ψm=Lm·Im.
Due to a saturation effect, the excitation inductance Lm changes with the air gap flux linkage ψm varying, thereby, excitation parameters with different excitation levels can be measured, and the excitation parameters form one group of excitation curves. The principle of calculating the excitation curve is as follows: a magnitude of the air gap flux linkage ψm depends on a voltage-frequency-ratio Us/f. In the no-load test, a nominal voltage and a nominal frequency are input into the induction motor, and then nominal excitation parameters ψmN, LmN and ImN can be calculated through the above method. A ratio of the nominal voltage and the nominal frequency can be considered as a benchmark, the benchmark is multiplied by different coefficients, and the alternating voltages obtained with the new ratios are input into the induction motor, and then multiple groups of excitation parameters (ψm, Lm and Im) can be calculated, and then these calculated excitation parameters are plotted as a curve, so as to obtain a curve of excitation parameters of the induction motor.
To sum up, in prior arts, the excitation parameters can be measured only when the induction motor is under no-load rotary operation. However, in many field applications, the induction motor has been connected to a load of a mechanical apparatus (e.g., a crane or a rolling mill, etc.), and then if the no-load test is implemented when the induction motor is connected to the load of the mechanical apparatus, the accuracy of measured excitation parameters will be severely affected by the load of the mechanical apparatus. But it is inconvenient for the operator to separate the mechanical apparatus from the induction motor in actual operations. Therefore, when the induction motor is connected to the load of the mechanical apparatus, a technical solution for measuring excitation parameters of the induction motor at standstill is desirable.